1. Field of the Invention
The present invention is in the field of power converters. The present invention is further in the field of semiconductor switching power converters. The present invention further relates to the field of integrated hysteretic non isolated control methods for switching power converters and circuits. The implementation is not limited to a specific technology, and applies to either the invention as an individual component or to inclusion of the present invention within larger systems which may be combined into larger integrated circuits.
2. Brief Description of Related Art
Modern electronic applications require power management devices that supply power to integrated circuits or more generally to complex loads. In general power switching converters are becoming more and more important for their compact size, cost and efficiency. The switching power converters comprise isolated and non isolated topologies. The galvanic isolation is generally provided by the utilization of transformers. The subject invention refers to non isolated power converters.
Modern switching power converters are in general divided in step down power converters also commonly known as “buck converters” and step up power converters commonly known as “boost converters”. This definition stems from the ability of the converter to generate output voltages that are lower or higher than the input voltage regardless of the load applied.
Boost converters can be implemented by storing and releasing energy in a passive component and more precisely in a capacitor or in an inductor. In particular the case of capacitive charging is also known as charge pump converter while, when the inductor is used, the converter is generally known as inductive boost converter.
Inductive boost converters are very important to generate well regulated voltage rails at voltages higher than the input voltage available. Typically, this is obtained by first charging the inductor with energy by applying a current through it and thereafter switching off a terminal of the inductor so as to discharge the current into a load at higher voltage. The most known and used prior art for a switching non isolated inductive boost converter topology is shown in FIG. 1.
The modes of operation of inductive switching power converters are mainly two. The first is the Continuous Conduction Mode (CCM) characterized by the fact that, at steady state, the inductor current increases and decreases with the switching frequency and duty cycle but it is never kept at zero during the duty cycle. CCM generally occurs when the load current is high enough to require a positive inductor current and therefore a constant flow of energy from the input to the output. If and when the inductor current crosses a zero value the converter can be kept in CCM by allowing the inductor current to become negative and therefore discharging the output capacitor.
The second mode of operation is the Discontinuous Conduction Mode (DCM) characterized by the fact that when the inductor current reaches zero value, it is kept at zero for part of the period. This second mode is generally entered to when the load current is small. If the load current is not very large the output capacitor can provide enough energy to the load for part of the switching period so that during that time interval the inductor energy can be null. Typically, in DCM the output voltage ripple is more pronounced since the energy is stored also in the output capacitor so as to allow lower switching frequency.
Fast control of boost converters is difficult to obtain in CCM because there is always an intrinsic delay in providing energy to the load since the inductor has to be first charged with current flowing in it. If the load suddenly changes from a low current to a high current load, the boost converter circuit has to spend some time to charge the inductor first and during this time no current/energy is supplied to the load. This phenomenon is not present in buck converters where by applying current to the inductor, the same current is flowing in the load as well.
The small signal analysis of the boost circuit in CCM points out to the presence of a right half plane zero (RHPZ). This is the effect that an increase of load current causes an apparently counter-intuitive decrease of the current in the diode due to an increase of duty cycle. This RHPZ can complicate the stability of the loop and generally is dealt with by rolling off the loop gain of the switching voltage regulator at relatively low frequency, making the overall response of the boost converter quite slow.
Generally the boost converters are controlled with PID (proportional-integral-derivative) type of control method. In particular current mode controls are quite common because they include two nested loops: one for the control of the output voltage and one for the control of the output current. However, as mentioned, these types of control methods do not present high bandwidth and require the adoption of large output capacitors to obtain acceptable load transient responses.
High frequency switching power converters are increasingly more popular due to the advantage of using low value inductors and capacitors reducing significantly the cost and board space of the power management section. Buck converters can successfully be operated at high frequency by using hysteretic and pseudo-hysteretic approaches. Generally the control loop of pseudo-hysteretic converters is simple and the output voltage is summed to a ramp signal to generate a synthetic ripple. A prior art example of pseudo hysteretic switching buck converter is provided in Klein (U.S. Pat. No. 7,457,140).
This signal is fed to a fast comparator that determines the charge and discharge timing of the inductor. For buck converters the implementation of a pseudo hysteretic control is relatively simple because the output stage of the buck, along with the inductor and the output capacitor, forms the integrating section of the converter that can be seen as a delta sigma converter. As mentioned, the buck converter charges the inductor while supplying current to the load.
The intrinsic delay of the boost architecture makes the implementation of an hysteretic approach much more difficult to obtain. However alternative approaches are possible. For instance a boost converter could be obtained by cascading a capacitor charge pump converter with an inductive buck converter. In that case the buck can be regulated with simpler and faster control loops to obtain high frequency operation and faster load transient responses.
The mentioned approach for the case in which the output voltage is smaller than twice the input voltage is illustrated in FIG. 2. The block 1 is the charge pump converter while the block 2 represents the inductive buck converter. For output voltages higher than 2× the input voltage, multiple stages of the charge pump circuit are needed. In these cases the charge pump is not regulated but simply operated to multiply the input voltage in an uncontrolled way as long as the provided output current is at least equal to the output load current.
However, due to the higher complexity of the circuit, its higher cost, the higher number of components and the lower overall efficiency, the applicants are not aware of any high volume industry adoption of the mentioned approach. An interesting observation is that if the capacitor C3 of FIG. 2 is removed, the input voltage to the regulated buck converter is already toggling between the boosted voltage and the input voltage (minus a diode D2 voltage drop).
Therefore the switches S4 and S5 of FIG. 2 can be omitted and if the inductor L2 is coupled between the output capacitor C4 and the boosting capacitor C2, as shown in FIG. 3, the output voltage of the overall circuit can be regulated approximately between 2*Vin and Vin by modulating the duty cycle of the switches S2 and S3. In this case, the boosting capacitor C2 has to be large enough to transfer enough energy during the boosting phase almost independently from the duty cycle. Since the inductor sees a voltage that toggles approximately between 2*Vin and Vin, the Vout/Vin transfer function is Vout=Vin+Vin*D, where D is the duty cycle of the converter.
A similar method but with simplified circuit topology was described in a paper by K. I. Hwu and Y. T Yau (“KY Converter and Its Derivatives” published on IEEE Transaction on Power Electronics—January 2009). The presented converter, in its simplest form, is again characterized by a Vout/Vin transfer function equal to 1+D where D is the duty cycle of the converter. However the control method used in the presented converter is a conventional PID, the switching frequency is very low (195 KHz) and most importantly it is meant to operate always in CCM for any load condition. These major limitations make this solution very unattractive for real applications and in particular for high frequency applications.
It is therefore a purpose of the present invention to describe a novel structure of a switching CL (capacitive-inductive) boost converter that operates at high switching frequency with pseudo-hysteretic control and operating both in CCM and DCM depending on the load conditions.